<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<script>
		let a = [1,2,3,4]
		let b = a.findIndex(function(i){
			return i>=3
		})
		
		let str = '12345'
		let c = a.indexOf(4)
		console.log(c)
		
		</script>
	</body>
</html>
